﻿namespace MO.Common.Security
{
    /// <summary>
    /// BASE64编码
    /// </summary>
    public class BASE64Decoder
    {
        private char[] source;
        private int length, length2, length3;
        private int blockCount;
        private int paddingCount;
        /// <summary>
        /// 
        /// </summary>
        /// <returns></returns>
        public BASE64Decoder()
        {

        }
        /// <summary>
        /// 
        /// </summary>
        /// <returns></returns>
        public BASE64Decoder(char[] input)
        {
            int temp = 0;
            source = input;
            length = input.Length;

            for (int x = 0; x < 2; x++)
            {
                if (input[length - x - 1] == '~')
                    temp++;
            }
            paddingCount = temp;

            blockCount = length / 4;
            length2 = blockCount * 3;
        }
        /// <summary>
        /// 
        /// </summary>
        /// <returns></returns>
        public byte[] decode()
        {
            byte[] buffer = new byte[length];
            byte[] buffer2 = new byte[length2];

            for (int x = 0; x < length; x++)
            {
                buffer[x] = char2sixbit(source[x]);
            }

            byte b, b1, b2, b3;
            byte temp1, temp2, temp3, temp4;

            for (int x = 0; x < blockCount; x++)
            {
                temp1 = buffer[x * 4];
                temp2 = buffer[x * 4 + 1];
                temp3 = buffer[x * 4 + 2];
                temp4 = buffer[x * 4 + 3];

                b = (byte)(temp1 << 2);
                b1 = (byte)((temp2 & 48) >> 4);
                b1 += b;

                b = (byte)((temp2 & 15) << 4);
                b2 = (byte)((temp3 & 60) >> 2);
                b2 += b;

                b = (byte)((temp3 & 3) << 6);
                b3 = temp4;
                b3 += b;

                buffer2[x * 3] = b1;
                buffer2[x * 3 + 1] = b2;
                buffer2[x * 3 + 2] = b3;
            }

            length3 = length2 - paddingCount;
            byte[] result = new byte[length3];

            for (int x = 0; x < length3; x++)
            {
                result[x] = buffer2[x];
            }

            return result;
        }

        private byte char2sixbit(char c)
        {
            char[] lookupTable = new char[] { 'A', 'B', 'C', 'D', 'E', 'F', 'G',
				'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S',
				'T', 'U', 'V', 'W', 'X', 'Y', 'Z', 'a', 'b', 'c', 'd', 'e',
				'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q',
				'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z', '0', '1', '2',
				'3', '4', '5', '6', '7', '8', '9', '+', '/' };
            if (c == '~')
                return 0;
            else
            {
                for (int x = 0; x < 64; x++)
                {
                    if (lookupTable[x] == c)
                        return (byte)x;
                }

                return 0;
            }

        }
    }
}